Lubricating oils are oils to be used for reducing friction between parts of a machine in contact with each other, and in general, for example, mineral oils, synthetic oils, animal and vegetable oils, and mixed oils thereof have been well known as base oils for the lubricating oils. Machines requiring lubricating oils are extremely large in number and cover a broad spectrum, and hence conditions under which the machines are used and performances which the machines are required to have are also various. Accordingly, the base oils are used appropriately depending on their applications. However, when a lubricating oil is used in an aircraft or a sophisticated hydraulic system, a hydraulic oil having a high flame retardant effect is required in some cases. A synthetic flame-retardant hydraulic base oil based on a compound that hardly burns, a water-containing flame-retardant hydraulic base oil obtained by incorporating water into a hydraulic base oil to improve its flame retardancy, or the like is generally used as a flame-retardant hydraulic base oil for such hydraulic oil. Examples of the synthetic base oil include a phosphate-based compound such as tricresyl phosphate (TCP) or triphenyl phosphate (TPP), and an ester-based compound containing a polyol and a linear saturated fatty acid (Patent Literature 1). In addition, examples of the water-containing base oil include a mixture system containing water and a glycol, a water-in-oil (W/O) emulsion system where water droplets are dispersed in oil, and an oil-in-water (O/W) emulsion system where oil droplets are dispersed in water (Patent Literatures 2 and 3).
However, phosphate-based compounds such as tricresyl phosphate (TCP) or triphenyl phosphate (TPP) have high toxicity and too low a viscosity to be used as a base oil, though the compounds have flame retardancy. Accordingly, concern has been raised about its load on the environment and need for limitations on the use of oils containing the compound. In addition, ester-based compounds containing polyols and linear saturated fatty acids have low toxicity but do not have sufficient flame retardancy. On the other hand, when a water-containing base oil is used, the base oil has low toxicity and is available at a low cost, but the fact that its maintenance and management are not easy is perceived as a problem. For example, base oils are lost due to water evaporation or are corroded by mold, bacteria, fungi, and the like. That is, at present, a high-performance flame-retardant base oil that is safer and more easily used as a base oil than the related-art products are being sought in the market.
Incidentally, the examples given above are examples of a flame-retardant hydraulic base oil, and the phosphate-based compounds such as tricresyl phosphate (TCP) or triphenyl phosphate (TPP) out of those examples are also well known to have an abrasion-preventing effect not as a base oil for lubrication but as an additive for lubrication (Patent Literature 4). However, as described above, such compounds have high toxicity and hence alternative compounds have heretofore been required in the field of additives as well. To meet the requirement, in recent years, phosphorus-based abrasion-preventing agent compositions for lubrication having low toxicity have started to be developed (Patent Literature 5) and are attracting attention.
Therefore, if a phosphorus-based compound having low toxicity that can be used as a flame-retardant base oil for lubrication and also as an additive for lubrication exhibiting abrasion resistance is developed, the usefulness and novelty of the compound would be extremely high, and hence the compound can be expected to be successful in many technical fields. Accordingly, the development of such a compound having not one function alone but multiple functions has been strongly demanded in the market because the compound provides merits on both the supply side and demand side in terms of efficiency and convenience. It should be noted that the phosphorus-based abrasion-preventing agent composition for lubrication described in Patent Literature 5 is an additive having low toxicity and good abrasion resistance. However, it is impossible to use the composition as a base oil because of its high viscosity. In addition, even if the composition is used as an additive, its mixability with a lubricant base oil may be poor owing to its high viscosity, and hence it may be difficult to handle the compound.